new rule - not calling super() in custom exception's __init__()

[m-aciek]

class CustomException(Exception):
    def __init__(self, custom: int, arguments: str) -> None:
        self.msg = f"Something specific happened with custom: {custom} and {arguments}"

Above exception class has an issue, in my personal experience it's quite common. The implementer assumes, that they will use the self.msg parameter during exception handling. The issue araises when the exception gets anywhere else as the its string representation is as follows:

>>> raise CustomException(42, 'test')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
__main__.CustomException: (42, 'test')

and when the exception is instantiated with keyword arguments, we lose the information about the arguments' values.

>>> raise CustomException(custom=1, arguments='two')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
__main__.CustomException

How to fix it? By calling super().__init__() in the constructor. That way the string representation of the custom exception will contain the error message.

class CustomException(Exception):
    def __init__(self, custom: int, arguments: str) -> None:
        self.msg = "Something specific happened with custom: {custom} and {arguments}"
        super().__init__(self.msg)
>>> raise CustomException(1, 'two')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
__main__.CustomException: Something specific happened with custom: 1 and two
>>> raise CustomException(custom=1, parameters='two')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
__main__.CustomException: Something specific happened with custom: 1 and two

Specification proposal

If a class:

• inherits from from Exception,
• doesn't implement own __str__() method,
• its __init__() method doesn't enforce any positional-only argument,
• and doesn't call super().__init__() in its __init__() method

Then:

• new rule should raise an error, calling to fix it.

[m-aciek]

For the reference, this has been accepted in flake8-bugbear.